The use of microwave energy to reheat or cook food products for consumption is increasing. Many such food products have a crust. By virtue of its manner of heating, microwave energy does not brown or crispen the crust.
It is well known, for example, from U.S. Pat. No. 4,641,005, that a continuous thin metallic film, generally vapour-deposited aluminum, may be employed to convert a portion of microwave energy incident thereon into thermal energy and that such thermal energy may be employed to effect heating of foodstuffs, particularly for the crispening and browning of outer crust material.
It is also well known that thicker metal films of foil thickness, such as aluminum foil, effectively act to reflect substantially all microwave energy incident thereon, and so act as a shield to prevent microwave energy from passing to the food. Metal film is of foil thickness generally have a thickness of about 0.0001 to about 0.01 inch, typically approximately 0.00035 inch for commercially-available aluminum foil.
There is described in U.S. Pat. No. 4,230,924 a structure which achieves browning of a microwave-cooked foodstuff and comprises a flexible dielectric wrapping sheet having a flexible metallic coating thereon, which may be in the form of aluminum foil. The coating is subdivided into a number of individual metallic islands by criss-crossing non-metallic gaps provided by exposed dielectric strips on the wrapping sheet. The flexible dielectric wrapping sheet is in the form of paperboard. (There is another embodiment disclosed in this reference in which the metallic coating is in the form of a thin film on a polymeric film substrate). In this arrangement, the individual metal foil islands are sized approximately 5/8 inch on a side and the criss-crossing, non-metallic gaps vary from about 0.001 to about 0.0625 inch in width. As described in the patent, the laminate is partially transparent to microwave energy, since microwave energy is permitted to pass through the gaps into a food product wrapped in the laminate to effect dielectric heating. The adjacent metallic islands are said to act as the plates of a capacitor to generate differences in electrical potential therebetween, which results in electrical current flow between the islands through the dielectric substrate. The islands are said to modify the microwave field configuration to achieve an enhance heating of the outer surface of the foodstuff, to effect browning and crispening.